Transcript Meteorites - Astronomy ITB

Meteorites
AS3141 Benda Kecil dalam Tata Surya
Prodi Astronomi 2007/2008
Budi Dermawan
Falls and finds (1)
• Meteorite find: typically, a farmer finds a strange
rocky/metallic object when ploughing his field
(most common in the museums)
• Meteorite fall: the fireball of the falling meteorite is
observed, and the freshly fallen pieces are found on the
ground
(useful for statistics of different types)
Falls and finds (2)
• The meteorites usually
fragment during flight; the
largest fragments travel
furthest along an oblique
in fall path
• The Antarctic ice forms
accumulation sites for
meteorites; these have
been explored recently
Meteorite find in the Lybian desert
Spectra
Classification
Meteorite types
• Chondrites
~85% of falls
- formed in the solar nebula
}
stony
• Achondrites
~8% of falls
- formed by igneous processes near the surface of major or
minor planets
• Stony irons
~1% of falls
• Irons
~6% of falls
- formed by fragmentation of core-mantle differentiated
asteroids
Meteorites that are finds are likely to be iron, because these are obviously
different from Earth’s rocks. Whereas the stony meteorites can blend in with
other rocks when viewed by untrained eye
Origin of Meteorites
 Radioactive dating puts ages at 4.6 Byr
 Meteorites originate in silicon and metal rich
meteoroids (asteroids), not the icy cometary
material that would burn up in the atmosphere
 Iron meteorites suggest molten cores. The
heat source would not have lasted long, and
this is consistent with a picture where the
meteorites formed early in the history of the
solar nebula
 Interactions with cosmic rays from the solar
wind alter or age the meteorites, but there
isn’t that much aging apparent, suggesting
that the meteorites must have been protected
under layers or rock until recently
 Meteorites originated relatively recently (<1
Byr) in collisions between asteroids or
planetesimals
Iron Meteorites
 Rare
 Interior generally shows complex structure
called Widmanstatten patterns formed from
iron-nickel alloys and the very high degree
of order requires that the molten metal
must have cooled extremely slowly (~20 K
every Myr)
 Must originate in the cores of meteoroids
large enough to be molten (to support
differentiation) and large enough to have a
significant insulating layer that leads to
very slow cooling of the molten core
Stony Meteorites
 Rich in silicates or stony materials
 The most common type is chondrite (from
the glassy inclusions called chondrules),
which have the same composition as the
Sun with all volatile gasses (H, He) missing
 Expected to be original samples of material
that condensed in the solar nebula
 Glassy chondrules are bits of melted rocks
that cooled too quickly to form ordered
crystalline structures
Chemical classes of chondrites
CI (Ivuna)
CM (Murchison)
CO (Ornans)
CV (Vigarano)
H (high iron)
L (low iron)
LL (low-low)
EH (high iron)
EL (low iron)
carbonaceous
~4% of falls
ordinary
~79% of falls
enstatite
~2% of falls
Structure of chondrites
• Matrix: dark, finegrained background
• Chondrules: nearly
spherical “droplets”,
typically of mm-size
• CAI (Calcium-Aluminumrich Inclusions) are
whitish, irregularly
shaped
Meteoritic
compounds
• Chemical equilibrium
reaction network of
solids in the solar
nebula
• Each mineral is marked
at the temperature
where it condenses or
sublimates
Chondrite formation
• Separation of high-and lowtemperature materials
• CAIs may result from
extreme heating in the early,
active nebula
• Chondrules were made by
rapid, less extreme heating
whose nature is not
understood
• Volatile depletion of matrix
remains to be explained
Chondrites as chronometers of
solar system formation
• Allende CAIs have Pb-Pb ages of 4560 Myr
• Whole-rock Pb-Pb ages of chondrites cluster around
4555 Myr
(207Pb enrichment due to U decay)
• Suggestion: CAIs formed during the early collapse
phase; chondrites were assembled a few Myr later in
a quiescent nebula
12C/13C
ratio in meteorites (1)
• Solar System average = 89.9
• The gas in the presolar cloud (mainly CO) was
homogenized
• The grains in the presolar cloud retained very
different ratios, reflecting various formation
environments
• Did such grains survive until they were
incorporated into chondrites?
12C/13C
ratio in meteorites (2)
• The answer is YES!
• The SiC grains are presolar and may be much older than the
Solar System
• Organic grains in 1P/Halley were found to range from 0.01 to
60, a still much wider range: presolar
Extinct radionuclides
Radio-nuclide
T1/2 (Myr)
Daughter species
26Al
0.7
3.7
6.5
16
103
82
26Mg
53Mn
107Pd
129I
146Sm
244Pu
53Cr
107Ag
129Xe
142Nd
fission Xe
Achondrites / parent bodies
• SNC meteorites (Shergotty, Nakhla, Chassigny)
come from Mars
• Lunar meteorites
• HED meteorites (Howardites, Eucrites,
Diogenites) come from (4) Vesta
• Ureilites come from a large carbonaceous
asteroid that is likely collisionally disrupted
Recent Results: Marchi et al. 2005 (1)
Flux of Meteoroid Impacts on Mercury
Model:
1. Meteoroid flux (radius r & impact velocity ):
Φ    ( , r )ddr   f ( , r )h(r )ddr
 (,r)  differential flux
f (,r)  differential normalized impact velocity distribution
h (r)  number of impacts
2. Delivery routes from MBAs are 3:1 & 6 resonances
(Morbidelli & Gladman 1998, Bottke et al. 2002)
Recent Results: Marchi et al. 2005 (2)
=1
Mercury
 is the ratio between 3:1 & 6 resonances
=5
 has only a little influence
Earth
Recent Results: Marchi et al. 2005 (3)
• Impacts on Mercury occur
from15 to 80 km s-1 (Earth 
50 km s-1)
• Impacts at perihelion
happen at considerably
greater velocity than
averaged over Mercury’s
entire orbit (47%, 43%,
33% for r = 10,000, 100, 1
cm)
Recent Results: Marchi et al. 2005 (4)
Impacts at aphelion have a symmetric distribution (am/pm = 1)
for r = 270 cm, while at aphelion is always am/pm > 1
 c (r )  1.4 r Myr
c is catastrophic
collisional half-time of
meteoroids that are
crossing the MBAs (r in
cm)
(Wetherill 1985, Farinella
et al. 1998)
Recent Results: Bottke et al. 2006 (1)
Iron meteorites as remnants of planetesimals formed in
the terrestrial planet region
Scattered into the mainbelt zone.
Once there the objects
are dynamically
indistinguishable from
the rest of the main-belt
population
Recent Results: Bottke
et al. 2006 (2)
o Enter the main-belt zone
through a combination of
resonant interactions and
close encounters with
planetary embryos
o Much of the particles is
delivered to the inner mainbelt, where most
meteoroids are dynamically
most likely to reach Earth
Recent Results: Bottke et al. 2006 (3)
 Inner solar system planetesimals experienced significantly
more heating than S- and C-type asteroids, with the most
plausible planetesimal heat source being radionuclides like
26Al and 60Fe
 If main-belt interlopers are derived from regions closer to the
Sun, their shorter accretion times would lead to more internal
heating and thus they would probably look like heavily
metamorphosed or differentiated asteroids
Recent Results: Bottke et al. 2006 (4)
Delivery efficiency of test bodies from various main-belt
resonances striking the Earth
Recent Results: Domokos et al. 2007
• Meteoroid flux at
Mars: <4.410-6
meteoroids km-2 h-1,
Masses > 4 g
• Flux at Earth: 10-6
meteoroids km-2 h-1
(Grün et al. 1985)
New mechanism of triggering meteorite
delivery to Earth
Yarkovsky thermal forces on Veritas family
The End
www.kosmochemie.de
iron meteorite with shiny fusion crust (width ca. 25 cm)